Analytic epidemiology investigates the causes of disease by determining why a disease rate is lower in one population group than in another. Hypotheses generated from descriptive data are tested and either accepted or rejected on the basis of results of analytic research. The epidemiologist seeks to establish a cause and effect relationship between a preexisting condition or event and the disease (see previous section on causality). To determine this relationship, two major types of research studies may be undertaken: observational studies and experimental studies.